1. Field of the Invention
The present invention relates to a process for recovering nitrogen in a low pressure type air separation apparatus provided with a reversing heat exchanger for removing water vapor and carbon dioxide gas from feed air, which is applicable to recovery of a product nitrogen gas under a predetermined pressure higher than the pressure of a nitrogen gas taken out of a low pressure column of duplex type rectification tower, but lower than the pressure of a nitrogen gas directly taken out of a high pressure column of the duplex type rectification column.
2. Prior Art
When a nitrogen gas from a high pressure column (which will be hereinafter referred to as "lower column") of duplex type rectification tower is used as a feed fluid for an expander in a low pressure type air separation apparatus using a reversing heat exchanger, the nitrogen gas is adiabatically expanded down to a pressure of a low pressure column (which will be hereinafter referred to as "upper column") of the duplex type rectification tower to increase the subcooling generation in the expander, and the effluent nitrogen gas from the expander is mixed with a low pressure product nitrogen from the upper column or a waste nitrogen gas according to the conventional process.
Recently a demand for nitrogen is increasing in various fields, and its application purposes are made versatile by employing new systems, etc. The pressure of a nitrogen gas recovered in the low pressure air separation apparatus is generally 100-500 mm Aq when the nitrogen gas is taken out of the upper column. When a nitrogen gas under a superatmospheric pressure is required, a nitrogen gas can be directly taken out of the lower column and supplied under a pressure of 4-5 kg/cm.sup.2 gage. However, in the latter case, it is necessary to increase the rate of feed air to meet the required pressure, and this is disadvantageous from the viewpoint of power requirement. Recently there is a demand for an air separation apparatus capable of recovering the nitrogen gas under a relatively low pressure of 0.5-2 kg/cm.sup.2 gage, depending upon application purposes.
To meet such a demand in the conventional low pressure type air separation apparatus, another nitrogen compressor is required in taking the nitrogen gas from the upper column, whereas the pressure becomes higher than required in directly taking the nitrogen gas from the lower column, resulting in an increase in power loss.
Furthermore in the low pressure type air separation apparatus using a reversing heat exchanger, it is necessary to control the temperature of return gas such as nitrogen gas, etc. to a specific temperature because of a sublimation problem of carbon dioxide gas in the reversing heat exchanger. For this purpose, two heat exchangers are provided at the outlet of the expander, where one of the heat exchangers serves as a heat exchanger for elevating the temperature of the effluent gas from the expander if it is lower than the inlet temperature of the reversing heat exchanger, and another serves as a heat exchanger for cooling if it is too high. However, the provision of two heat exchangers and switching a high temperature fluid to a low temperature fluid or vice versa to control the temperature of the effluent gas from the expander are not only expensive from the viewpoint of apparatus, but also complicated from the viewpoint of automatic temperature control, and thus this system has not actually been employed yet.
According to one actually employed example, another passage is provided at a liquefier in the waste nitrogen line from the upper column and employed without any automatic control on the basis of the fact that the saturation temperature of air in the lower column is not changed. This system is applicable to the apparatus of not so large scale without any problem, but in the case of the apparatus of large scale, a heat transfer area must be increased because of the gas-gas heat exchange in the heat exchanger, and thus the system is disadvantageous in cost.